1. Field of the Invention
The present invention relates to a method of magneto-optically recording erasing information subjected to overwrite access and a magneto-optical information storage medium which employs the method.
2. Description of the Related Art
A thin film made of rare earth-transition metal amorphous alloys (to be referred to a-RE-TM hereinafter) such as TbFe, TbCo, TbFeCo, GdTbFe, GdTbFeCo has an easy magnetization axis in a direction perpendicular to its surfaces. Magnetic properties of this film changes in accordance with temperature. Magneto-optical recording utilizes the above properties. More specifically, a recording layer made of an a-RE-TM is irradiated with a laser beam, and the temperature of the recording layer is increased to decrease its coercive force to a level lower than an external magnetic field. A small reversed-magnetic domain is formed in this portion by the external magnetic field, thereby recording information. The small reversed-magnetic domain is caused to disappear to erase information. In this manner, information is reproduced by using a magneto-optical effect such as polar Kerr-rotation. Magneto-optical recording has advantages in non-contact accessibility, compatibility of media, and a high recording density, which are common advantages in optical recording. However, magneto-optical recording has a disadvantage in overwriting as compared with magnetic recording.
Various types of overwriting techniques in magneto-optical recording have been proposed. The most attractive one of them is of the light modulation and one beam type (which is called one beam method).
The one-beam method is described in Published Unexamined Japanese Patent Application No. Sho 62-175948. In this publication, an initial auxiliary magnetic field is applied to a magneto-optical recording medium having a recording/reproducing layer and an auxiliary recording layer to align the direction of magnetization of the auxiliary recording layer in a predetermined direction. The magneto-optical recording medium is irradiated with a modulated beam pulse. When a beam intensity is high, a bit having one direction of magnetization is formed. However, when the beam intensity is low, a bit having the other direction of magnetization is formed.
According to this technique, since the recording/reproducing layer is exchange-coupled with the auxiliary recording layer, the exchange coupling force which is must be controlled to have a specified magnitude with respect to a coercive force of each layer which changes with a change in temperature. Therefore, it is difficult to manufacture the medium. In addition, prior to overwrite operation, the direction of magnetization of the auxiliary recording layer must be aligned in a predetermined direct ion. For this purpose, the initial-magnetizing magnet for applying a magnetic field of several kOe must be large in size, thus posing technical problems.
Another conventional technique disclosed in Published Unexamined Japanese Patent Application No. Sho 62-154347 is related to a medium obtained by stacking a recording layer and a bias layer having a compensation point, in which a nonmagnetic layer having a magnetical insulation property is interposed therebetween. According to this technique, the medium is designed such that the bias layer has different temperatures in the write and erase modes. The bias layer must be designed to differentiate the directions of saturated magnetization Ms of the bias layer at those two different temperatures. Therefore, it is difficult to manufacture such a medium.
The light-modulated one-beam method has received a great deal of attention as an overwrite technique in magneto-optical recording. However, in this method, it is difficult to manufacture a medium and the structure of the medium is complicated. Therefore, no practical magneto-optical recording method using this technique has been proposed.